The achievement unlocks after you
reduce the 100th enemy to a pile of ash when killing it with a plasma weapon. Plasma weapons are denoted with a red icon in the upper right hand corner of their info panel. The earliest plasma weapon you are likely to come across is the Bolter Pistol from an enemy drop on Emerald Vale or the Plasma Carbine purchased from the T&L vending machine at the landing pad next to the town of Edgewater.
Plasma weapons can be ranged as in the previous two examples or melee like the plasma cutter. They can deliver plasma damage as an inherent feature as in the above examples. Alternatively, a ranged weapon can be converted to a plasma weapon with the Mag-2-Power mod while a melee weapon can be converted with a Mr. Power mod. All plasma weapons have the potential to reduce the target to a pile of ash but will not do so on every kill. Only kills resulting in the pile of ash count towards the achievement.
If you use a plasma weapon, you will unlock the weapon through normal gameplay. The Hunting Rifle is an enemy drop early in the game. It can be converted to a plasma weapon with a Mag-2-Power mod acquired from an enemy drop or purchased from a vendor or vending machine. It is an excellent weapon for long range kills and has a decent probability of converting the target to ash.
If you do not use plasma weapons and/or want to get the achievement out of the way early, you can grind it.
The following is a recommended approach to unlock the achievement early in the game. It works for any difficulty and any character build):
Note 1: All kills made in sneak mode count towards the following achievement and you can grind for it as well:
--Proceed through the game until you reach the town of Edgewater. Avoid killing sprats. Continue to play until you acquire a plasma weapon. In my experience the Bolter Pistol is the one most likely to be found first. Alternatively, acquire either a Mag-2-Power or Mr. Power mod that you can use to convert a ranged or melee weapon respectively to a plasma weapon using the workbench on your ship or elsewhere in Emerald Vale.
--Once you have a plasma weapon or the wherewithal to make one, return to your ship. Equip a plasma weapon.
--Sleep till night-time. Make a manual save on the ship. Make note of it and don't over-write it until after you get the achievement.
--Return to Edgewater. Enter the main entrance and immediately exit again. This banks an auto-save. If you are playing on other than supernova difficulty, you can make a manual save just outside the main entrance instead of relying on the autosave. If making a manual save, do not overwrite the save you made on the ship.
--Double-check to make sure a plasma weapon is equipped and it is the active weapon.
--Turn left as soon as you exit back out of town and circumvent the perimeter of the town counter-clockwise. Note a Spacer's Choice poster on the wall to your left that says, "Taste the Freedom" and turn the corner.
--Shortly after you turn the corner, you will come across a number of sprats. They will be asleep and are not easy to spot. The first is near the base of a big vertical pipe running down a structure to the right. The remaining four are on an approximate line between that sprite and the corner to your front left. Crouch and approach each one.
--Optionally, invoke tactical time dilation (TTD),
, for extra damage and kill each sprat (
).
--After you have killed the 5 sprites, proceed around the perimeter to the far side of town wrt the main entrance just around the corner of what is depicted as the 'junkyard' on your map. There is no need to go into the junkyard.
--Kill another 6 sleeping sprats while crouching.
--After killing as many of the 11 sprats as is convenient, reload the auto-save you banked when you entered town.
--Repeat the process killing another 11 sprats or so and reload the banked auto-save.
--Continue killing sprats around town's perimeter & reloading the banked auto-save when you run out of targets.
--Some sprats are well hidden and hard to find If you have trouble spotting them, ignore them and reload the banked auto-save before you reach the 11 count.
--When you have reduced the 100th sprat to ash, the achievement will pop. Similarly, if applicable, once you get 50 sneak kills, the
Never Seen achievement will also pop.
Note 2: If you, accidentally or intentionally, lose the banked auto-save (or manual save made outside the Edgewater entrance), reload the manual save you made on the ship. You will retain all progress towards the achievement. Return to town, make another banked auto-save at the entrance by entering & immediately exiting. Resume progress towards the achievement by (sneak) killing sprats around the perimeter.
Note 3: The sprats can also be killed during the day but they will be active. They are very quick so killing them is more of a challenge and it is harder to get sneak kills.